drm/omap: dss: Remove the dss_mgr_(dis)connect() operations

The dss_mgr .connect() and .disconnect() are implemented as no-op in
omapdrm. The operations are unneeded, remove them.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>